1. Field of the Invention
The present invention relates generally to a heat treatment system and method for heat-treating an object to be treated.
2. Description of Related Art
First, the art related to a first invention provided by the present application will be described below.
As methods for carrying a large number of semiconductor wafers (which will be hereinafter referred to as wafers) in a batch type furnace to oxidize a silicon film on each of the wafers to form a silicon oxide film (SiO2 film), there are known a dry oxidation method using oxygen (O2) gas and hydrogen chloride (HCl) gas, and a wet oxidation method for feeding water vapor and oxygen gas into a reaction tube. The type of the oxidation method is selected in accordance with the quality of an intended film.
In the dry oxidation method, a silicon film is oxidized with oxygen gas, and impurities on the surface are removed by the gettering effect of chlorine. Specifically, for example, after a large number of wafers are held on the shelves of a boat to be carried in a vertical heat treatment furnace to form a treatment atmosphere of a predetermined temperature, oxygen gas and chlorine gas are supplied into a reaction tube from the ceiling portion of the heat treatment furnace at ordinary temperatures, and exhausted from the bottom side. In the wet oxidation method, an external combustion system must be provided outside of a heat treatment furnace. In the external combustion system, part of oxygen gas, and hydrogen (H2O) gas are burned to produce water vapor, and the rest of oxygen gas and water vapor are supplied to a reaction tube. In the above described heat treatment furnace, dinitrogen oxide gas (N2O gas) is fed into a reaction vessel at ordinary temperatures to be allowed to react with a silicon layer on the wafer to produce a nitrogen containing silicon oxide film.
By the way, a defect called slip is more easily caused in the wafer as a process temperature rises. Therefore, in order to avoid the influence of heat on films stacked on an underlayer and in order to save energy, it has been studied to lower the process temperature. However, if the process temperature is lowered, the uniformity of the thickness on the surface of the wafer deteriorates combined with the increase of the diameter of the wafer, and the variation in thickness between wafers (between planes) increases.
After the relationship between the thickness of a silicon oxide film obtained by the dry oxidation method and the position of a wafer on the boat was examined, it was found that the uniformity of the thickness of the film tended to deteriorate as the wafer was positioned on the upper stage side of the boat. The inventor guesses that the reason for this is as follows. FIGS. 6(a), 6(b) and 6(c) schematically show the flow of gases over a wafer W and the temperature and thickness of the wafer W. Oxygen gas and chlorine gas flow from the peripheral edge of the wafer W to the center thereof, and silicon on the wafer is oxidized with oxygen gas. Since heat of the wafer W is radiated from the peripheral edge of the wafer W, the temperature of the wafer W is higher at the center. Thus, the oxidation reaction is accelerated at the center, so that the thickness of the film at the center tends to be originally larger than that at the peripheral edge even if the uniformity of the thickness is high.
On the other hand, hydrogen produced by the decomposition of hydrogen chloride reacts with oxygen to produce a very small amount of water vapor. Since the gases are not sufficiently warmed on the upper stage side of the boat, the amount of produced water vapor increases as the gases are heated from the peripheral edge of the wafer W toward the center thereof. This water vapor serves to increase the thickness of the oxide film, so that the difference in amount of produced water vapor has a great influence on the thickness of the film. As a result, the distribution in thickness is a so-called crest distribution wherein the thickness of the film at the center of the wafer W is large, so that the uniformity of the thickness deteriorates. Then, since the gases are warmed as the gases travel toward the bottom of the reaction tube, the water vapor producing reaction is substantially in an equilibrium state on the lower stage side of the boat, so that water vapor is completely produced before the gases flow along the wafer W. Therefore, when the process gases flow from the peripheral edge of the wafer W toward the center thereof, the amount of water vapor hardly varies regardless of the position of the wafer W, so that the uniformity of the thickness of the film is enhanced. It is thus considered that the uniformity of the thickness of the film is low on the upper stage side of the boat so as to increase the difference in thickness of the film between wafers on the upper and lower stage sides.
Also in the process for producing a nitrogen containing silicon oxide film using dinitrogen oxide gas, the same tendency appears if the process temperature is lowered. That is, dinitrogen oxide gas is decomposed to allow oxygen to react with silicon to produce the nitrogen containing silicon oxide film, and the active species of nitrogen produced by the decomposition of the dinitrogen oxide gas enters the silicon oxide film to grow the nitrogen containing silicon oxide film. The temperature of the wafer W is higher at the center of the wafer W, and the dinitrogen oxide gas is not sufficiently decomposed on the upper stage side of the boat if the process temperature is low. Therefore, as the dinitrogen oxide gas flows toward the center of the wafer W, the decomposition reaction is further accelerated at the center of the wafer W than the peripheral edge thereof, so that the thickness of the film at the center of the wafer W tends to be larger than that at the peripheral edge thereof. Since the dinitrogen oxide gas is warmed as it travels toward the bottom of the reaction tube, the decomposition thereof sufficiently proceeds on the lower stage side of the boat, or the decomposition thereof further proceeds on the lower stage side of the boat than the upper stage side thereof even if it is not sufficient. Thus, the difference in degree of decomposition between the center and peripheral edge of the wafer is small. As a result, the inplane uniformity of the thickness of the film is higher than that on the upper stage side.
Thus, in the present circumstances, the inplane uniformity of the thickness of the film on the wafer is low on the upper stage side, and the uniformity between wafers is low, so that it is difficult to lower the process temperature.
The first invention provided by the present application has been made in such circumstances, and it is an object of the first invention to provide a technique capable of obtaining the high uniformity of the thickness of an oxide film, and contributing to the lowering of a process temperature, when an oxidation process is carried out with respect to an object to be treated.
The art related to a second invention provided by the present application will be described below.
There is a process called the CVD (Chemical Vapor Deposition) as one of deposition processes which are processes for fabricating semiconductor devices. This technique is designed to feed a process gas into a reaction tube to deposit a thin film on the surface of a semiconductor wafer (which will be hereinafter referred to as a wafer) by a chemical gas phase reaction. As one of systems for carrying out such a deposition process in a batch, there is a vertical heat treatment system. As shown in, e.g., FIG. 11, this system comprises a vertical reaction tube 112 provided on a cylindrical manifold 111, a heater 113 provided so as to surround the reaction tube 112, a gas feed pipe 114 extending into the manifold 111, and an exhaust pipe 115 connected to the manifold.
In such a system, a large number of wafers W are held on the shelves of a holder 116 called a wafer boat to be carried in the reaction tube 112 from an opening which is formed in the bottom end of the manifold 111, and a process gas is fed into the reaction tube 112 from a gas supply source 117 via the gas feed pipe 114 to deposit a thin film. At this time, the process gas is heated by the heater 113 in the reaction tube 112 to be decomposed, and further heated to a reaction temperature or higher to carry out a predetermined reaction. The reactant is deposited on the wafer W to form a predetermined film thereon.
By the way, if the film is deposited on the wafer W by means of the above described system, the thickness of the film in the central portion of the wafer tends to be larger than that in the peripheral edge portion thereof as shown in FIG. 12. It is considered that the reason for this is as follows. That is, in the above described vertical heat treatment system called a so-called batch type furnace, a process gas is fed into the reaction tube 112 from the gas feed pipe 114 to be supplied from the peripheral edge portion of the wafer W to the wafer W, which is held on the wafer boat 116, to flow along the wafer from the peripheral portion of the wafer to the central portion thereof, so that the concentration of the process gas in the central portion of the wafer is higher than that in the peripheral portion thereof.
In a process for raising the temperature of the wafer to a process temperature, the heat radiation amount in the peripheral edge portion of the wafer W is greater than that in the central portion thereof, so that the temperature of the central portion of the wafer is higher than that of the peripheral portion thereof. Thus, it is guessed that the deposition reaction is further accelerated in the central portion of the wafer W, in which the temperature and concentration of the process gas are higher, than the peripheral portion thereof due to the differences in temperature and concentration of the process gas between the peripheral and central portions of the wafer W, so that the thickness of the film on the central portion of the wafer W is larger than that on the peripheral portion thereof.
On the other hand, in a semiconductor fabricating process, in order to prevent a bad influence on a film produced at the last step and in order to save energy, a low temperature process is desired. However, the above described phenomenon that the thickness of the film increases in the central portion of the wafer tends to be conspicuous as the process temperature is lowered, so that it is difficult to realize a low temperature process in the existing system.
Therefore, the inventor has studied a technique for lowering a process temperature in the reaction tube 112 by preheating a process gas to a predetermined temperature by means of a heater (not shown), which is provided outside of the reaction tube 112, before feeding the process gas into the reaction tube 112, and feeding the activated and sufficiently heated process gas into the reaction tube 112. For example, the heater comprises a heating chamber for heating a fed process gas, and a heater, provided outside of the heating chamber, for heating the heating chamber. In this technique, since the process gas is preheated by the heater to a temperature approximating to, e.g., a decomposition temperature, the sufficiently activated process gas is fed into a deposition region, and a reaction sufficiently occurs when the process gas reaches the peripheral edge portion of the wafer. Thus, the reaction state in the central portion of the wafer is the same as the reaction state in the peripheral edge portion thereof, so that it is possible to provide the high uniformity of the thickness of the film even if the process temperature in the reaction tube 112 is low.
However, in a low pressure CVD process for reducing the pressure in the reaction tube 112 to carry out a process, the pressure in the heater is also reduced. If the pressure in the heater is reduced to, e.g., about 200 Torr, convection is difficult to occur. In addition, if the pressure in the heater is low, the partial pressure of the process gas decreases, so that heat conduction due to the convection of the process gas in the heater is difficult to occur. Thus, heat is not transferred into the interior of the heater, and the efficiency of heat transfer to the process gas is bad, so that it is difficult to heat the process gas to a temperature at which the process gas is sufficiently activated.
The second invention provided by the present application has been made in such circumstances, and it is an object of the second invention to provide a heat treatment system and method capable of obtaining the high uniformity of the thickness of a formed film and contributing to the lowering of a process temperature by supplying a process gas preheated by a heating part to a reaction vessel, for example, when a thin film is deposited on an object to be treated.